Early endocytosis as a key to understanding mechanisms of plasmalemma tension regulation in filamentous fungi

Abstract

Two main systems regulate the plasmalemma tension and provide a close connection of the protoplast with the cell wall in fungi: turgor pressure and actin cytoskeleton. These systems work together with the plasmalemma focal adhesion to the cell wall and their contribution to fungal cell organization has been partially studied, but remains controversial in model filamentous ascomycetes and oomycetes, and even less investigated in filamentous basidiomycetes. Early endocytosis, in which F-actin is actively involved, can be used to research of mechanisms regulating the plasmalemma tension, since the latter influences on the primary endocytic vesicles formation. This study examined the effects of actin polymerization inhibitors and hyperosmotic shock on early endocytosis and cell morphology in two filamentous basidiomycetes. The main obtained results: (i) depolymerization of F-actin leads to the fast formation of primary endocytic vesicles but to inhibition of their scission; (ii) moderate hyperosmotic shock does not affect the dynamics of early endocytosis. These and a number of other results allowed offering a curtain model of regulation the plasmalemma tension in basidiomycetes. According to this model, the plasmalemma tension in many nonapical cells of hyphae is more often regulated not by turgor pressure, but by a system of actin driver cables that are associated with the proteins of focal adhesion sites. The change in the plasmalemma tension occurs similar to the movement of the curtain along the curtain rod using the curtain drivers. This model addresses the fundamental properties of the fungal structure and physiology and requires confirmation, including through the yet technically unavailable high quality labeling of the actin cytoskeleton of basidiomycetes.